1. Field of the Invention
The invention relates to an image forming apparatus.
2. Description of Related Art
FIG. 21 of the accompanying drawings schematically shows the construction of an electrophotographic type laser beam printer which is an example of a conventional image forming apparatus.
In this example, a drum-shaped electrophotographic photosensitive member, i.e., a photosensitive drum 1, as an image bearing member is rotated at a predetermined speed in the direction of arrow. The surface of the photosensitive drum 1 is charged by a charging roller 2 as charging means for effecting primary charging so that the surface potential thereof may become uniform. On the uniformly charged photosensitive drum 1, a laser beam 3 is ON/OFF-controlled by exposure means on the basis of inputted image data and is scanned, whereby an electrostatic latent image is formed on the photosensitive drum. The electrostatic latent image formed on the photosensitive drum 1 is visualized as a toner image by the developer of developing means 4.
On the other hand, a sheet feeding cassette 26 stacks thereon recording materials, usually recording sheets P which are recording mediums, and feeds a recording sheet P to the location of registration rollers 24 by the driving of a sheet feeding roller 22.
The toner image visualized on the photosensitive drum 1 is transferred to the recording sheet P under the action of a transferring roller 5 as transferring means. Any toner residual on the photosensitive drum 1 is removed by cleaning means 7, and the photosensitive drum 1 is used for the next image forming.
The photosensitive drum 1, the primary charging means 2, the developing means 4 and the cleaning means 7 are integrally made into a cartridge which can be easily interchanged with respect to an apparatus main body 100 by a user.
The toner image transferred to the recording sheet P is heated and pressurized by fixing rollers 6 (fixing means) and is fixed on the recording sheet P. The recording sheet P having had the toner image thereon fixed is discharged onto a sheet discharge tray or the like.
The above-described conventional image forming apparatus, however, has suffered from a problem as shown below.
That is, in an image forming process, the recording sheet P is transported to a transferring region at which the transferring roller 5 is provided, and the toner image formed on the photosensitive drum 1 is transferred to the recording sheet P. When the transfer of the toner image is finished up to the trailing edge of the recording sheet P, the recording sheet P is separated and transported from the photosensitive drum 1.
When the trailing edge of the sheet is separated from the photosensitive drum 1, a transfer bias is applied to the transferring roller 5 and therefore, stripping discharge occurs between the photosensitive drum 1 and the trailing edge of the recording sheet. If for example, a plus voltage is applied as the transfer bias, the memory of a discharge trace will remain on the photosensitive drum 1 due to the stripping discharge, and as shown in FIG. 7 of the accompanying drawings, a trace will be produced as a lateral black line on the next page.
According to the result of our studies and experiments, to prevent such black line, it has been effective to switch off the transfer bias before the trailing edge of the sheet separates from the photosensitive drum 1. Thereby, the stripping discharge itself occurring when the recording sheet P separates from the photosensitive drum has been mitigated and the black line has been improved.
By this countermeasure, however, the photosensitive drum 1 does not receive the plus voltage of the transfer bias within a range in which the transfer bias has been rendered off and thus, only that portion does not receive a transfer memory. Therefore, only that portion of the photosensitive drum 1 which has not received the transfer voltage becomes somewhat high in the surface potential thereof though it is generally of the order of −500 to −600 volts. Thereby, as shown in FIG. 8 of the accompanying drawings, there has arisen a new problem that the density of the next page becomes low only at the pertinent location. This problem is particularly remarkable when an image on a page on which an image is to be formed next time is of a half tone.